damnedifidon't, again I don't really have any advice regarding mixed feeding; we only give our 8mo old DD the odd bottle), but I will agree that like others on here it took me a while to get into my stride with it, and I finally had my BF epiphany at around 12 weeks where suddenly eveything just clicked and started to go right.
6 weeks is another typical growth spurt time and the going can get really tough, but you are doing great for keeping up with it and you shouldn't let yourself forget that
Like you, I have older kids (well one, a five year old) and after my DH went back to work when DD was three and a half weeks old, I was basically on my own as he works away through the week. the school run took on a new dimension and my five year old felt the dethronement very keenly and his behaviour was a nightmare. Add to that the school holidays when DD was six weeks old and I was a nervous wreck.
I felt like giving up at every turn, especially when recurrent thrush, blocked ducts and mastitis were added to the mix, but now at eight months I'm glad I kept going. We started to introduce the odd bottle of formula around four months, just so I could get a bit of a break as I've never managed to express very successfully.
Now it all seems so easy. I can't imagine having to make up formula and allow it to cool first thing in the morning, whilst trying to organise my DS for school, with a hungry screaming baby on my arm too. so much easier to latch her on and sit with a cup of tea when DS is eating his breakfast.
However, saying all of that, you have to do what feels right for you and your family, at the moment. you have enough pressures on your life it would seem without the added anxiety that the breastfeeding is causing you. Someone mentioned on another thread that the guilt gene kicks in when you have children, aND IT SOUNDS LIKE IT AFFECTING YOU BIG TIME AT THE MOMENT. (oops, sorry for shouting ). Yes you are feeling guilty about thinking of stopping, but you do have other children to consider in this.
Sorry, I'm a bit long-winded, and not much help on the mixed feeding (although a friend of mine did it successfully, but not sure the ins and outs of when she fed what), but I hope some of that helped. And fwiw, I think that whenever one decides to stop BFing, there will always be that element of guilt, always the thought that one should have done it for longer.
You will get great support on here whatever you decide